Understanding the causes of neurodegenerative diseases with the aim of developing effective treatments is a significant medical challenge. Much effort has been expended identifying susceptibility genes for amyotrophic lateral sclerosis (ALS) and Alzheimer's (AD) disease. A number of genes have been found, each explaining a minor percentage of familial cases and contributing little to our understanding of the pathogenesis of sporadic cases. However, one common feature of many neurodegenerative diseases is protein aggregation, and it is widely thought that revealing the processes of cytoplasmic aggregate formation is key to better understanding the disease. We propose a novel hypothesis: rather than mutations at single gene locus, increased expression of ubiquitous LINE1 (L1) retrotransposons in the brains of ALS or AD patients, perhaps from many locations in the genome, contribute to the disease pathology. We predict that the consequent increase in cellular levels of the L1-encoded ORF1p RNA-binding protein could increase its binding to proteins of significance for ALS or AD and promote their sequestration in stress granules and other cytoplasmic aggregates. Furthermore, a corresponding increase in L1 ORF2 protein could account for increased levels of reverse transcriptase activity that have been detected in sera of ALS patients. LINE1 retrotransposons are non-viral mobile DNA elements that duplicate themselves by a copy and paste mechanism using an RNA intermediate. The Human Genome Project estimated that over 500,000 L1 copies occupy 17% of human DNA, although it is believed that only about 100 of these remain potentially active in any individual. L1 retrotransposition has also been responsible for the insertion of over a million non- autonomous Alu retroposons and thousands of processed pseudogenes. The cell in turn has evolved defenses against unrestricted retrotransposition, including DNA methylation, chromatin remodeling, nucleic acid editing, and RNA interference. However, recent investigations show that these defenses are occasionally relaxed, leading to increased retrotransposon activity in certain somatic cell types, including stem cells, some cancers, and notably neuronal cells in the human brain. Using immunocytochemistry, RNA analyses, coimmunoprecipitation, and a functional assay for L1 ORF2 reverse transcriptase activity, we will examine brain tissue and derived iPS cell lines, mainly from ALS patients but also including AD samples, and compare these with age-matched controls to ascertain if L1 RNA and protein expression is elevated in the disease state. We will assay for alteration in the size, number, and morphology of L1 ORF1p-mediated cytoplasmic RNA granule formation in patient samples and monitor for association and colocalization of ALS/AD-associated proteins with the L1. Common mutations in both ORF1p and disease susceptibility proteins will be assayed for their effects on coaggregation.